Known propulsion units can include a casing, an electric motor, a shaft, and a propeller. The casing can be rotatably attached to a hull of a vessel. The electric motor can be positioned within the casing. The shaft can pass through the electric motor and can be rotatable supported with bearings within the casing. The propeller can be attached to an outer end of the shaft protruding from the casing. In propulsion units where a high thrust at low speeds is used there can be an annular housing surrounding the perimeter of the propeller. The annular housing can be attached to an upper portion of the casing and to one or several vanes being supported at the casing. The annular housing can form a central duct with an axial flow path for water from a first end to a second end of the annular housing. The thrust produced by the propeller can be amplified by the housing at low speeds. The situation can be such that the propeller can produce 60% of the total thrust and the housing can produce 40% of the total thrust at low speeds. Housings can be used, for example, in Dynamic Positioning (DP) vessels used in oil drilling. Several propulsion units can be used in such vessels and the vessel can be kept steady in position by the propulsion units. For example, a big thrust can be used at low speed to keep the vessel continuously in position in rough seas. In addition, the housing can increase the thrust produced by the propeller at low speeds, for example, at speeds in the order of less than about 15 knots.
WO patent publication 99/14113 discloses one example of a propulsion system including a casing, an electric motor, a shaft, a propeller, and an annular housing surrounding the perimeter of the propeller. The system is intended for vessels moving in ice conditions. The system can include a strut rotatably supported at the hull of the vessel, a torpedo-shaped casing attached to the strut, a drive shaft and an electric motor within the casing, a propeller attached to an outer end portion of the drive shaft, and a nozzle surrounding the propeller. The nozzle can have a water inlet and a water outlet. Rotatable blades or vanes can be attached to an outermost end of the drive shaft outside the water inlet of the nozzle for breaking and/or crushing ice before the ice enters into the nozzle.
In known propulsion units including a casing, an electric motor within the casing and a propeller outside the casing driven by the electric motor, a lack of torque at low speed can exist. The diameter of the electric motor within the casing can be limited and the also the length of the electric motor within the casing can be limited. The housing surrounding the outer perimeter of the propeller can increase the thrust of the arrangement at low speed.
Known propulsion units can be based on rim drive arrangements. A rim drive propulsion unit can include a rotor positioned on the outer periphery of the propeller and a stator positioned within an annular housing surrounding the propeller. The diameter of the rotor and the stator of the electric motor can be increased, wherein the torque of the electric motor can also be increased. The propeller can be attached to a shaft being supported by bearings on both sides of a radial plane passing through the axial middle point of the rotor and the stator. The shaft and the bearings can be within the flow path in the interior of the propulsion unit.
U.S. Patent Application Publication No. 2012/0093668 discloses a rim driven thruster including an annular housing, a propulsion assembly, a magnetic rotor assembly and a stator assembly. The annular housing defines a flow path extending along an axis. A conical forward fairing can be connected to the front end of the housing and a cylindrical aft fairing can be connected to a rearward end of the housing. The propulsion assembly can be supported within the housing and can include propeller blades extending radially from the axis of the flow path. The propeller blades can be configured to rotate about the axis. The magnetic rotor assembly can be mounted to radially outer ends of the propeller blades. The stator assembly can include spaced propeller drive modules mounted to an inner circumferential surface of the annular housing. The propeller drive modules can be configured to provide electromagnetic torque to the magnetic rotor assembly. There can be a shaft extending along the axis in the middle of the annular housing and support brackets at both ends of the annular housing extending in the radial direction from the inner surface of the annular housing towards the axis in the middle of the annular housing. The shaft can be rotatably supported by bearings at both ends to the support brackets.
U.S. Pat. No. 6,837,757 discloses a rim driven propulsion arrangement. The arrangement can include a housing supported by a strut from an adjacent vessel, which can be propelled by the propulsion unit. The housing can form a duct with an axial flow path for water. The inner surface of the duct can be conical at the aft end of the housing. A cylindrical rotor support assembly can be situated centrally within the housing and supported at an aft end by an array of support members in the form of blades at the housing. A rotor assembly can include a hub and radial propeller blades attached to the hub. The hub can be rotatably supported with radial bearings at a central stationary support shaft within the rotor support assembly at a forward end of the rotor support assembly. The axial bearing, for example, the thrust bearing can include a thrust ring rotating with the rotor at the forward end of the hub and a stationary thrust plate affixed to the rotor support assembly. Water can be used as lubrication medium in the thrust bearing.
JP Patent Publication No. 10257752 discloses a drive unit for watercraft. The drive unit can include a central shaft rotatably supported with bearings within a cylindrical casing and an annular housing surrounding the cylindrical casing. A hub can be positioned on the central shaft, the hub having propeller blades attached radially thereto. A coupling unit can be attached to the outer edges of the propeller blades. Several rotor coils can be arranged on the coupling unit. Stator coils can be positioned in the annular housing surrounding the casing. Either the stator coil or the rotor coil can be made of superconducting material and the other coil can be made of conducting material. The propeller and the central shaft can be rotated by supplying current to the stator coil to generate an electromagnetic force between the stator coils and the rotor coils.
Known rim drive propulsion units can have a radial plane passing through the axial middle point of the rotor that coincides with a radial plane passing through the axial middle point situated between the radial bearings of the central shaft.
A rim drive propulsion unit can produce more torque at low speed due to the increase in diameter of the rotor and the stator. The lubrication of the bearings of the shaft supporting the rotor of the rim drive propulsion unit in known units can be based on special arrangements designed for the application. These special arrangements can be based on the use of water as the lubrication medium. Water lubrication, however, can cause corrosion problems and the lubrication properties of water can be limited. Seawater can be corrosive, such that fresh water can be used for lubrication. However, separate fresh water tanks have to be built in the vessel for use of fresh water for lubrication means.